1. Field of the Invention
The present invention generally relates to a method for forming a film pattern.
2. Description of the Related Art
Methods for forming a pattern in photolithography can be classified broadly into etching methods and liftoff methods. An etching method is a method for forming an objective element and circuit, whereby a film of a material for the element and circuit is formed on the substrate beforehand; then, a mask is formed thereon by carrying out patterning by use of a resist; unnecessary portions of the film are eliminated by etching through the mask; and, thereafter, the resist is eliminated. The etching method includes a dry etching method and a wet etching method, each of which may be suitably chosen.
In contrast, a liftoff method is a method whereby a film of a material for the element and circuit is formed on the resist, which had been applied on the substrate and was patterned; thereafter, unnecessary portions of the film on the resist are eliminated together with the resist.
The two methods for forming a pattern have their own merits and faults but generally, in terms of precision, the etching method may be more effective and, in terms of cost, the liftoff method may be more effective.
Whether a positive resist or a negative resist is used, it is possible to carry out either the liftoff method or the etching method therewith.
Positive resist is a material in which the portion exposed to light is dissolved and the unexposed portion remains insoluble. Negative resist is a material in which the portion exposed to light is cured and the pattern of the exposed portion remains. When patterning the resist, a developer is used and, when eliminating the unnecessary resist, a remover is used. In general, the components of the developer and remover are different.
A commonly used positive resist includes DNQ (diazonaphthoquinone) and a novolac resin.
First, a mechanism of development involving DNQ will be described. In the portion exposed to light, DNQ loses nitrogen and, via a keto-carbene intermediate, generates a ketene. This ketene reacts with moisture possessed by the novolac resin and is transformed into 3-indene carboxylic acid. This indene carboxylic acid is soluble in an alkaline aqueous solution and facilitates dissolution of the novolac resin in the alkaline aqueous solution. On the other hand, DNQ is hydrophobic and, when added to the novolac resin, it suppresses dissolution of the resin in the alkaline aqueous solution. This difference in solubility and the transition from dissolution suppression to dissolution enhancement enable positive patterns.
This basic concept can also be applied to chemically amplified resists.
Next, the mechanism of removal involving DNQ will be described. In the portion unexposed to light, DNQ is hydrophobic and is insoluble in alkali. Thus, it is necessary to use a solution containing as the main component an organic solvent which can dissolve the mixture of DNQ and a novolac resin. Therefore, the developer and remover for the positive resist cannot be made of the same material, due to different solubility requirements of the developing and removing steps.
Negative resist will be described with a cyclized rubber-bisazide resist as an example.
When a photosensitive aromatic bisazide is blended into a cyclized rubber and exposed to light, a crosslinking reaction occurs. When the light exposure exceeds a certain level, this crosslinking results in formation of a three-dimensional network structure and the cyclized rubber becomes difficult to dissolve in the organic solvent, thus providing a negative resist. From the cured portion (the portion exposed to light) and uncured portion (the portion unexposed to light), a pattern of the portion having reduced solubility in the organic solvent can be obtained. As for removal of the resist, use of a strongly polar organic solvent which can better dissolve the cured portion enables removal. Thus, in the case of a negative resist, the developer and remover are also not prepared from the same material.